Bridge design errors blamed

November 14, 2008

WASHINGTON (AP) -- Safety investigators on Thursday singled out undersized steel plates as the chief cause of last year's deadly collapse of a highway bridge in Minneapolis. But contractors working on the bridge had stockpiled construction material on the center span over the Mississippi River, and that additional weight contributed to the collapse that killed 13 people and injured 145, they said. Federal investigators told the National Transportation Safety Board that the collapse on Aug. 1, 2007, of the Interstate 35W bridge was unavoidable once gusset plates in the center span failed. When that happened, it dragged other sections of the bridge and rush-hour commuters into the water. The plates helped connect the bridge's steel beams. Board members criticized Minnesota transportation officials for allowing the storage of 287 tons of construction materials for lane-widening on the bridge. The materials were stored above the gusset plates that fractured. But board members said it was not possible to determine if the materials alone -- or factors such as weather and traffic, combined with the added weight -- pushed the plates to a breaking point. "Had the gusset plates been properly sized, this bridge would still be there," said Bruce Magladry, director of the NTSB's office of highway safety. Investigators said the half-inch thick plates were inadequate to handle traffic and other stress factors and did not meet engineering guidelines when the bridge was built in 1967. The safety board, as far back as January, had identified design flaws in the plates as a critical factor in the collapse. The board's final ruling was expected today.